Conventionally, improvements in flatness and surface roughness are very important for a glass plate employed for substrates of semiconductor devices, spacers for field-effect flat panel displays, or substrates of magnetic disks. However, a float process or a casting process currently typically used as a method of manufacturing a glass plate produces glass plates with low flatness when used to manufacture thin glass plates. Therefore, the glass plate has to be finished to an appropriate flatness for the above use through grinding and polishing of a significant amount of a surface thereof. As a result, the glass plate after the grinding has an extremely unfavorable surface roughness.
To solve the problem as described above, the ground glass plate typically is subjected to the polishing twice, so that the surface roughness is 0.5 nm after the first polishing, and approximately 0.1 nm after the second polishing. It is expected that a third polishing will be required in addition to the above, since there will be a demand for a product with higher precision in the next generation. Therefore, an endeavor to improve the flatness of the glass plate only through the grinding and polishing will end up in more time and work for grinding and polishing, which eventually leads to a higher equipment cost.
In view of the above, a method is devised to manufacture a thin glass plate of a desirable thickness using a base material glass plate with a predetermined thickness and an improved surface roughness and by heating the base material glass plate to soften the same and drawing the softened glass plate (see patent Document 1).
Further, in connection with the above method of manufacturing a glass plate, a technique for eliminating local unevenness of the thickness of a glass plate is disclosed, according to which, plural coolers are arranged along a width direction of the glass plate, and power of a heater placed at a position corresponding to an uneven portion is adjusted, or the glass plate is cooled partially in the width direction (see Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-Open No. H11-199255    Patent Document 2: Japanese Patent Application Laid-Open No. H8-183627